An R-T-B based sintered magnet (where R is at least one of rare earth elements, indispensably containing Nd, and T is a transition metal element, indispensably containing Fe) that contains a Nd2Fe14B compound as a main phase is known as a magnet with the highest performance among permanent magnets. Such a magnet is used in various motors for hybrid vehicles, electric vehicles, home appliances, etc.
The R-T-B based sintered magnet has its coercivity HcJ (hereinafter simply referred to as “HcJ” in some cases) reduced at high temperatures, leading to irreversible thermal demagnetization. For this reason, R-T-B based sintered magnets applied in electric motors for hybrid vehicles or electric vehicles, which are potentially used in a relatively-high temperature environment, are required to have high HcJ values.
Conventionally, to improve HcJ, a large amount of heavy rare earth element (mainly Dy) is added to the R-T-B based sintered magnet. However, this results in a problem that a residual magnetic flux density Br (hereinafter simply referred to as “Br”) is reduced. Because of this, in recent years, a method for obtaining high HcJ while suppressing the reduction in Br has been used in which a heavy rare earth element is diffused from the surface into the inside of a R-T-B based sintered magnet, causing the heavy rare earth element to be concentrated on an outer shell of the main phase crystal particle.
Dysprosium (Dy) has various issues, including inconsistent supply and large fluctuations in price because of restrictions on its resources. For this reason, a technique has been required to improve HcJ of an R-T-B based sintered magnet while reducing the use of heavy rare earth elements, such as Dy, as much as possible.
Patent Document 1 describes that the B content is set lower than that in the standard R-T-B based alloy, while at least one element selected from Al, Ga, and Cu is contained as a metal element M to thereby form an R2T17 phase, thus ensuring an adequate volume ratio of a transition metal-rich phase (R6T13M) generated using the R2T17 phase as a raw material, whereby an R-T-B based rare-earth sintered magnet with high coercivity can be produced while reducing the Dy content. Patent Document 2 describes that the B content is set lower than that in the standard R-T-B based alloy, while the contents of B, Al, Cu, Co, Ga, C, and O are controlled within predetermined ranges, and further an atomic ratio of Nd and Pr to B and an atomic ratio of Ga and C to B respectively satisfy specific relationships, which achieves the high residual magnetic flux density and coercivity.